Now, liquid crystal display elements are used widely. Among them, a TN (twist nematic) type display mode is most widely used as a low-grade display element. The said TN display mode has many advantages such as low driving voltage, low consumed electric power and so on. As to a response speed, however, it is much inferior to luminous type display elements such as cathode-ray tube, electroluminescense and plasma display elements.
Although a display volume for a liquid crystal display element has been improved eminently owing to development of the so-called STN display element which is a new type TN display element and its twist angle made 180.degree..about.270.degree., there is still a limit about the response speed thereof. Furthermore, display elements equipped with switching elements on respective pixels of the TN display elements have appeared recently on the stage of the market. Much of them are thin film transistor elements (abbreviated as TFT type), and they have occupied much of the market as liquid crystal elements with high density, high volume and full colors.
However, since the said mode has a high dependence property on a visual field angle, it is preferably used for a personal terminal but unsuitable for an all-directional display. With the object of compensating the disadvantage, there has been developed an in-plane driving (abbreviated as IPS driving) display mode which is superior in visual angle characteristic.
In spite of these improvements, there are generally mentioned disadvantages about an image size and a production costs for TFT display. Since TFT uses a semiconductor technique, the image size thereof has a limit of twenty inch order and a time division ability has also a limit of 1000 lines.
On the other hand, display elements with using ferroelectric liquid crystals or antiferroelectric liquid crystals have been actively developed.
As a ferroelectric liquid crystal display (SSFLC) mode of a surface stabilized type with using the former liquid crystals was proposed by N. A. Clark and S. T. Laggawall in 1980. The latter antiferroelectric liquid crystalline phase was found by Furukawa et al. in 1987 at first and tentatively named as a chiral smectic Y (SY*) phase (see Ferroelectronics, Vol 85, p451, 1998). Thereafter, Chandanni et al. proved that the said phase is an antiferroelectric liquid crystalline phase. (see Japanese Journal of Applied Physics, Vol 28, p1265, 1989).
Both are the display modes for dissolving a dependence property of displayed colors on a visual field angle including reversal of tone, which is an essential defect of TN type display. Furthermore, they are expected for decrease in an element preparation cost since displaying in cell structure for a simple matrix is possible even during high time division driving.
In ferroelectric liquid crystalline phase states, only two states are stable, wherein an inclining direction of liquid crystalline molecules is determined by an applied direction of an electric field. In sum, one of two states can be selected by polarity of voltage.
In the antiferroelectric liquid crystalline phase states, there is also the third state stable during non-applied period of an electric field in addition to the above-mentioned states for the ferroelectric liquid crystalline states. The said state during non-applied period of the electric field is the so-called antiferroelectric state.
The non-electric field state (the antiferroelectric state) is subject to phase transition to ferroelectric phase state by application of the electric field and returned to the antiferroelectric phase state by removal of electric field. Two stable positions (a bistable stae) are present during application of the electric field (a ferroelectric phase state), and only one stable position is present without the electric field (antiferroelectric phase state). Three states including them are utilized for switching.
Hitherto, it was said that switching properties between three states have sharp threshold properties and exhibit wide optical hysteresis. There have been, however, reported recently those without any clear threshold in phase transition due to the electric field of the ferroelectric-antiferroelectric state and also without any observed optical hysteresis (for example, Young Member Society for Liquid Crystalline Research, the third lecture meeting, S6-1 etc.).
Recently observed features thereof are that antiferroelectric liquid crystals have not any memory action in principle and that they are very preferable for trials to fill them into a TFT type cell having an accomplished manufacturing process now and apply active matrix driving. The feature that a clear threshold being not appeared means the possibility for display of all half gradations, which suggests realization of full color displays.
Combination of wide field visual angle properties of ferroelectric liquid crystals and antiferroelectric liquid crystals in the TFT structure is expected to dissolve the visual field angle dependence, which is the largest weak point of liquid crystals.
In the case that antiferroelectric liquid crystals are used for switching elements etc., particularly in the case that application for the above-mentioned TFT driving being conducted, driving voltage is required to be low. Driving voltage for IC used in practical TFT elements is about 5V, which could not be attained by the conventional ferroelectric liquid crystals and antiferroelectric liquid crystals hitherto. Although many compounds having antiferroelectric liquid crystal properties have been reported, there are few antiferroelectric liquid crystalline compositions having low driving voltages and enough wide active temperature ranges. It is caused by the fact that only few antiferroelectric liquid crystalline compounds for constituting the compositions having enough satisfied properties are present.
The subject solved by the present invention is to provide an antiferroelectric liquid crystalline compound suitable for low voltage driving, more preferably for TNT driving.